At present, radiation detection techniques including a direct conversion technique and an indirect conversion technique are known. In the direct conversion technique, pulse height of the output is proportional to the energy of incident radiation are output. In the indirect conversion technique, fluorescence produced when radiation is incident on a scintillator is detected by a photodiode, a photomultiplier element, and the like.
In the indirect conversion technique, a scintillator has a property that the number of fluorescence photons emitted by the phosphor is proportional to the radiation energy incident on the phosphor. Thus, counting the number of fluorescence photons emitted by a phosphor enables measurement of the energy of radiation having passed through a subject. This property can be applied to a computed tomography (CT) system and the like to acquire a CT image through energy discrimination, for example, and allow material decomposition on the CT image. Furthermore, if y-ray energy emitted when a radioactive isotope disintegrates can be detected, a spatial distribution of radioactive isotopes dispersed on the ground, for example, can be obtained.
In radiation energy decomposition in combination of a scintillator and a photon detector, for energy discrimination through calculation of the number of fluorescence photons emitted by the scintillator, the output pulse generated by detection of a radiation need to be integrated for a predetermined time.
In development of radiation measuring apparatuses based on a technique (photon counting technique) of measuring radiation photons in a single photon region and analyzing the energy of the radiation photons, higher count rates of detecting elements and highly accurate energy decomposition are desired.
In the conventional technique, pulses having pulse heights proportional to radiation energy are generated, a plurality of comparators having thresholds corresponding to the number of decomposition stages are set for the pulse heights corresponding to the radiation energy are provided, and the comparators measure the frequencies of occurrence of pulses with the respective thresholds to form a histogram for radiation energy.
With the conventional technique, however, comparators, the number of which is proportional to the required number of energy decomposition stages, need to be implemented, which is an obstruction to increasing the density and reducing the power consumption of radiation measuring apparatuses.